Birds must choose between mating and migrating, study finds


By Communications Staff
Wednesday, April 6, 2011

Sex or nice weather. That's the agonizing choice some birds face, according to a new study being published today in the Royal Society’s Biology Letters Journal.

Researchers from The University of Western Ontario and the University of Guelph discovered that some male birds migrating to lower elevations, where lighter rainfall makes it easier to forage for food, may be unable to attract a female when they return home.
Western Postdoctoral Fellow Alice Boyle and Western Biology Professor Chris Guglielmo, working with Professor Ryan Norris at the University of Guelph and Research Scientist Keith Hobson at Environment Canada, examined the breeding behaviour of the white-ruffed manakin. This small Costa Rican bird is partially-migratory, choosing each year whether to migrate or stay.
The researchers discovered the males that choose to stay behind gain better breeding sites and are more likely to increase or maintain their standing within the population, making them more attractive to females.
Most animals migrate because of the cold, but in the tropics some populations are partially-migratory. These species provide a window into the evolution of migrating and a unique opportunity to examine the costs and benefits of this behaviour.  The study is the first to investigate the pros and cons of staying or going in a partially-migratory species.
"The manakins are faced with the choice of boosting their chances of survival or boosting their chances of breeding," says Boyle. "Birds that are younger are more likely to migrate, because they still have several years of breeding opportunities ahead of them. So, it's often the older birds, with a higher status that stay, not only because there is a sense of urgency to breed, but also they know staying will improve their chances of becoming an alpha."
During two breeding seasons, the researchers tagged almost 200 birds to observe their status and mating success. Alpha males in the population often have the most prominent breeding sites and perform the most elaborate aerobatic displays to attract the females.
To learn which birds had migrated approximately 15 kilometres to lower elevations prior to breeding season, the scientists tested samples of their claws. It was discovered that migrating birds have more heavy hydrogen in their claws. Rain at the lower elevation contains more of these isotopes, which end up in plants and berries eaten by the manakins. These isotopes of hydrogen then become fixed in their nails.

Boyle observes, "The problem with studying migration is that it's hard to follow the animals around, but this method allowed us to go back in time to find out what each bird was doing before we captured it."
The researchers found the males that did not migrate had better breeding sites, a higher status and attracted more females than the migrating males.
Media Contacts:
Alice Boyle, The University of Western Ontario, 519-661-2111 ext. 84669; 
Ryan Norris, University of Guelph, 519-824-4102 ext. 56300; 
Maureen Spencer Golovchenko, Communications & Public Affairs, 519-661-2111 ext. 85165;  














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